726 5. Tonzig and E. Marre 



induced increase of this compound was, in most cases, within what 

 can be considered a physiological range (20 to 100 per cent increase). It 

 is well known, in fact, that large changes of the AA level may occur 

 under physiological conditions, as a function of light intensity or of 

 temperature (3). The inhibiting effect of AA treatment on cell elonga- 

 tion was confirmed by experiments in which geotropic and phototropic 

 bending of stems and coleoptiles, as well as bending caused by uni- 

 lateral application of auxin (indole-3-acetic acid, lAA), was inhibited 

 by the application of a lanolin-AA paste to the shoot bases (21). 



In further experiments Tonzig and Bracci have shown that in some 

 cases cell division can also be markedly inhibited by treatment with 

 AA. In fact, no root nodules developed on the roots of pea plants in- 

 fected with Rhizohium leguminosarum when AA at concentrations 

 ranging from 0.1 to 0.05 per cent was added to the medium. Similarly, 

 treatment with AA prevented the effect of the bacteria on the induc- 

 tion of calluses in wounded pea stems (19). 



On the whole, these experiments on intact plants have shown tliat 

 an increase of the internal level of AA effectively inhibits growth 

 in a number of species and under various experimental conditions. 

 Moreover, experiments in which AA changes in the different plant 

 parts were determined confirm that AA is easily translocated in the 

 plant, translocation upward being much more rapid than transloca- 

 tion downward. 



Isolated Plant Parts 



Growth by elongation of etiolated pea internode segments and 

 Avena coleoptile sections appeared strongly inhibited by AA at con- 

 centrations higher than 10^ M (12,20). Very low concentrations of 

 AA occasionally induced a slight stimulation, an effect which has 

 been recently investigated by Chinoy et al. (I). 



The fact that the stimulating effect of AA at very low cencentra- 

 tion ajjpears relatively weak and scarcely reproducible, when com- 

 pared with the much more consistent inhibitory effect of AA concen- 

 trations high enough to induce a significant increase of the internal 

 level of this compound, seems to indicate that the endogenous con- 

 tents of AA in the materials investigated are very close to or even 

 higher than the concentration rccjuired for maximal gro\\th. This 

 condition obviously makes the study of the growth inhibiting com- 

 ponent more susceptible to experimental investigation. On the other 

 hand, there seems to be no ground to assume that the inhibitor) com- 

 ponent of the AA effect has a less important physiological role, in the 

 intact plant, than the eventual stimulating component. 



